Transdermal and/or transmucosal delivery of active agents provide a convenient, pain-free, and non-invasive method of administering active agents to a subject. Additionally, the administration of active agents, such as drugs, through the skin or mucosal surface avoids the well-documented problems associated with the “first pass effect” encountered by oral administration of active agents. As known in the art, orally administered drugs are absorbed and enter the bloodstream where they are transported by the portal vein directly to the liver before entering the general circulation of the body. If the drug is subject to a high hepatic clearance, i.e., it is rapidly metabolized by the liver, then a substantial fraction of the absorbed dose is extracted from the blood and metabolized before it ever reaches the systemic circulation. A consequence of this “first pass effect” phenomenon is a significant reduction in the bioavailability of the drug. In some instances, the first pass effect is so large as to render oral administration of a drug ineffective. Furthermore, all the orally absorbed drug is concentrated in the portal vein producing a much higher concentration in portal blood than the corresponding systemic levels. In the case of steroidal hormones, including estrogens, the primary concern of the higher portal vein concentrations of the drug is a greater dose-dependant genomic effect on liver protein synthesis than the transdermal and transmucosal delivery systems. For a period of time, in the order of months, these very high hormone levels in the portal vein quickly and significantly stimulate the production of higher circulating levels of thrombogenic clotting protein factors and free fatty acids. This phenomenon is not seen to the same extent with transdermal administration as the hormone absorbed transdermally reaches the liver at the lower systemic hormone level eliminating this fast increase in protein and fatty acid production.
Although the transdermal and/or transmucosal delivery of active agents overcome some of the problems associated with oral administration of active agents, such as that described above, they are not free of their own drawbacks. Problematically, transdermal drug delivery systems are typically restricted to low-molecular weight drugs and those with structures having the proper lipophilic/hydrophilic balance. High molecular weight drugs, or drugs with too high or low hydrophilic balance, often cannot be incorporated into current transdermal systems in concentrations high enough to overcome their impermeability through the stratum corneum. Specifically, polar drugs tend to penetrate the skin too slowly, and since most drugs are of a polar nature, this limitation is significant.
Efforts have been made in the art to chemically modify the barrier properties of skin to permit the penetration of certain agents (since diffusion is primarily controlled through the stratum corneum), enhance the effectiveness of the agent being delivered, enhance delivery times, reduce the dosages delivered, reduce the side effects from various delivery methods, reduce patient reactions, and so forth.
In this regard, penetration enhancers have been used to increase the permeability of the dermal surface to drugs, and are often protons accepting solvents such as dimethyl sulfoxide (DMSO) and dimethylacetamide. Other penetration enhancers that have been studied and reported as effective include 2-pyrrolidine, N,N-diethyl-m-toluamide (Deet), 1-dodecal-azacycloheptane-2-one N,N-dimethylformamide, N-methyl-2-pyrrolidine, calcium thioglycolate, hexanol, fatty acids and esters, pyrrolidone derivatives, derivatives of 1,3-dioxanes and 1,3-dioxolanes, 1-N-dodecyl-2-pyrrolidone-5-carboxylic acid, 2-pentyl-2-oxo-pyrrolidineacetic acid, 2-dodecyl-2-oxo-1-pyrrolidineacetic acid, 1-azacycloheptan-2-one-2-dodecylacetic acid, and aminoalcohol derivatives, including derivatives of 1,3-dioxanes, among others.
The penetration enhancers used and thought to be necessary to transdermally deliver active agents such as steroid hormones, namely, compounds such as long chain fatty acids such as oleic acids, fatty alcohols such as lauryl alcohol and long-chain fatty esters such as isopropyl myristate, tend to include aliphatic groups that make the formulations oily and malodorous.
For example, U.S. Pat. No. 5,891,462 teaches the use of lauryl alcohol as a permeation enhancer for estradiol and norethindrone acetate. Such formulations are not appealing to the user nor to anyone else in close proximity to the user. Although this particular patent discloses three examples of estradiol or norethindrone acetate formulations having no lauryl alcohol component, such formulations are comparative examples that are intended to illustrate the long held position that long chain fatty alcohols such as lauryl alcohol are necessary to transdermally deliver norethindrone acetate in combination with estradiol to a subject.
Additionally, for example, the known testosterone gel formulations FORTIGEL® and TOSTRELLE® (Cellegy Pharma, South San Francisco, Calif.), both include ethanol, propanol, propylene glycol, carbomer, triethanolamine, purified water, and oleic acid as a permeation enhancer, the latter being responsible for the irritating and malodorous characteristics of these formulations. Also, TESTIM® (Auxilium Pharmaceuticals, Norristown, Pa.) is a 1% testosterone gel and includes pentadecalactone, acrylates, glycerin, polyethylene glycol (PEG), and pentadecalactone as a permeation enhancer. It is a very odoriferous compound. Also, TESTIM® is not desirable because it contains undesirable amounts of glycerin which are not well tolerated by the skin.
Thus, there is a need for a transdermal formulation that adequately delivers active agents to patients with skin tolerability, but does not include the unpleasant odor common to the prior art formulations.